PFDisplacedVertexFinder is effectively disabled in standalone PAT.py/miniAOD configuration

[slava77]

standalone PAT.py , e.g.
cmsDriver.py step3 --conditions auto:run2_mc --era Run2_2016 --eventcontent MINIAODSIM --runUnscheduled -s PAT --datatier MINIAODSIM
does not import (or loses otherwise PFDisplacedVertexFinderCandidateProducer particleFlowDisplacedVertexCandidate)
as a result, this collection can not be produced in the standalone PAT.
Downstream, PFDisplacedVertexProducer (particleFlowDisplacedVertex) is implemented in a way that silently ignores missing inputs
https://github.com/cms-sw/cmssw/blob/CMSSW_8_1_0_pre15/RecoParticleFlow/PFTracking/src/PFDisplacedVertexFinder.cc#L53
So, PFDisplacedVertexFinder is effectively disabled: it runs but will produce an empty collection.
(Full RECO+miniAOD workflows are not affected

particleFlowDisplacedVertex is used in MET corrections.
It's unclear how long has this bug been present.
@mmarionncern please check the effects of this issue.